Field of the Invention
The present invention concerns a superconducting magnet coil arrangement, as well as a method for reducing a quench pressure within a cryogen vessel that contains a superconducting magnet coil arrangement.
Description of the Prior Art
In a superconducting magnet assembly for MRI applications, a liquid cryogen such as helium is often provided within a cryogen vessel which also contains superconducting coils supported on a mechanical retaining structure such as a former. During a quench event, as is well known, energy stored in the superconducting coils is dissipated as heat causing boiling of the cryogen. The boiling of the cryogen increases the pressure within the cryogen vessel, known as quench pressure, until a pressure-limiting device such as a valve or a burst disc opens to provide a gas egress path at a certain quench pressure.
The standard approach for magnet design is to minimize the increase in coil temperature during a quench, and to design a large supporting former which mechanically supports and retains the coils, and also acts as a heat sink for the magnet coils. During a quench event, heat from the coils is conducted to the former, which is typically of aluminum or stainless steel. This limits the rise in the temperature of the surface of the coils and the former in contact with the cryogen.
Conventional superconducting magnets are cooled down to about 4K using liquid helium to induce a superconducting state. The magnet coils are ramped to a specified electric current, which has an associated stored energy. When a superconducting magnet undergoes a transition from the superconducting state to the normal/resistive state, as in a quench, any stored electrical current is transferred from the superconductor filaments into the copper cladding typically provided around superconducting filaments. An amount of heat is generated by Ohmic heating of the magnet coils. The heat from the magnet coils is then transferred via thermal conduction into the former and the liquid helium, both of which are in thermal and mechanical contact with the coils.
The amount of energy dissipated and the rate at which the stored energy is transferred from the magnet coils into the liquid helium, together with the volume of helium and the geometry of the pressure vessel containing the magnet and helium determine the quench pressure within the helium vessel. For example, the design of the helium vessel and the available turret venting path cross-section will influence the fluid impedance experienced by escaping cryogen gas. High quench pressures are undesirable because of the need to increase the pressure vessel wall thickness, and therefore cost and weight, to cope with such pressures and the need to increase the cross-sectional area of the turret to relieve the quench pressure.
Increased turret area will increase its thermal heat load into the helium vessel, which results in the requirement of increasing the cooling power required from an associated cryogenic refrigerator. It is preferred to minimize the required cooling power, for reasons of cost.
Current superconducting magnet designs use the parameters of operating current and number of turns—which determines the energy stored in the magnet coils, quench propagation circuit properties, vent path area, and vessel strength to engineer a solution for managing the quench pressure.
FIG. 2 schematically represents a conventional cylindrical magnet structure with superconducting coils 20 wound onto an aluminum former 22, which acts as a heat sink. A radially outer surface 26 of the superconducting coil 20 directly contacts liquid cryogen, and forms the main interface for heat transfer from the coil to the liquid cryogen to cool the coil.
Conventionally, during a quench, radially outer surface 26 is at a temperature To, typically about 80K, while the radially inner surface 28 of the former 22 is at a temperature Ti, typically about 20K. Heat flows Q1, Q2 are shown, where Q1 represents heat flux from coil 20 to former 22, while Q2 represents heat flux from coil 20 to adjacent cryogen. Some conventional arrangements dispense with the former, thereby providing more effective cooling of the radially inner surface of the coils due to increased contact surface area between coils and cryogen.
The present invention provides a superconducting magnet in which the magnet coil structure comprises a resistive element that will control the surface temperature of the magnet coil structure, so controlling the rate of heat dissipated into the liquid cryogen. In preferred embodiments, the superconducting magnet does not have the conventional former, which acts as a heat sink.
The resistive element is provided as a secondary coil of insulated resistive wire mechanically attached to a surface of the superconducting coil which will control the rate of cryogen boiloff caused by the quench, in turn determining the quench pressure for a given cryogen vessel and quench path exit, which in turn determines the required strength of the cryogen vessel. By reducing the rate of heat transfer to the cryogen, the required volume of cryogen may be reduced, the peak quench pressure may be reduced and so the cryogen vessel may be made of a thinner material, and/or the quench path exit may be reduced in size, in turn reducing a source of heat influx into the cryogen vessel.